Projection display apparatuses using various types of spatial light modulators conventionally are known as large-screen image equipment. Recently, a reflection-type spatial light modulator with high display efficiency such as DMD (digital micro-mirror device) has been receiving attention (e.g., JP 2000-98272 A).
FIGS. 16A and 16B show the configuration of a projection display apparatus using a DMD as a spatial light modulator. FIG. 16A is a top view of the apparatus, and FIG. 16B is a side view thereof. This projection display apparatus includes the following: a lamp 161 for emitting white light; an elliptical mirror 162 for condensing the emitted light of the lamp 161; a rotating color filter 164 that is located in the vicinity of a long focus of the elliptical mirror 162 and selectively transmits three primary colors (red, green, and blue) of light in sequence; a focusing lens 165; a plane mirror 166; a DMD 167 for modulating incident light to form an optical image; and a projection lens 168 for magnifying and projecting the optical image formed on the DMD 167 onto a screen (not shown).
As the lamp 161, e.g., a super-high pressure mercury lamp or xenon lamp may be used. These lamps provide high brightness with a relatively small light-emitting portion, so that the emitted light can be condensed efficiently. The focusing lens 165 suppresses the divergence of light that has passed through the rotating color filter 164 and directs the light toward the DMD 167 and the projection lens 168.
FIG. 17A is a schematic front view of the DMD 167. FIG. 17B is a schematic side view showing the principle of operation of small mirrors 171 on the DMD 167. As shown in FIG. 17A, the DMD 167 includes a two-dimensional array of small mirrors 171 that are provided for each pixel. The inclination of the individual small mirrors 171 is controlled by the electrostatic effect of memory devices located directly under the small mirrors 171 so that a reflection angle of the incident light is changed for each pixel, thereby producing the ON/OFF states.
FIG. 17B illustrates a condition in which a small mirror 171 is inclined at ±10 degrees with respect to the plane of the DMD 167. For incident light 172 that tilts 20 degrees from a normal to the plane of the DMD 167, when the small mirror 171 is in the ON (+10 degrees) state, reflected light 173 enters the projection lens 168, and a pixel is displayed on the screen. When the small mirror is in the OFF (−10 degrees) state, reflected light 174 does not enter the projection lens 168, and a pixel is not displayed on the screen. It is possible to express the gray scale by temporally controlling the ON/OFF switching of each pixel.
Each of the mirrors 171 on the DMD 167 is rotated in a plane that forms an angle of 45 degrees with a minor axis 176 of the display area (this angle is referred to as “bearing angle” in the following), as shown in FIG. 17A.
To ensure appropriate operation of the DMD 167 and to avoid mechanical contact or interference between the optical components such as the focusing lens 165 and the projection lens 168, the plane mirror 166 is arranged so that the optical path of light emanating from the focusing lens 165 bends three-dimensionally and the light enters the DMD 167 at a predetermined incident angle, as shown in FIGS. 16A and 16B.
A central axis (a normal passing through the center of an effective portion in the DMD 167) 167a of the DMD 167 does not coincide with an optical axis 168a of the projection lens 168, but is offset (shifted) from the optical axis 168a. Therefore, the projection lens 168 uses only part of the field angle of an image circle for projecting an optical image formed on the DMD 167.
However, the projection display apparatus as shown in FIGS. 16A and 16B has the following problems.
First, the central axis 167a of the DMD 167 is offset from the optical axis 168a of the projection lens 168, so that excess space is necessary in the height direction, making it difficult to reduce the size of the whole apparatus.
Second, when this apparatus is used in a rear-projection display apparatus, the optical axis 168a of the projection lens 168 is offset from the central axis of a projected image. Therefore, the central axis (a normal passing through the center of an effective portion of a screen) of a transmission-type screen that is held by a cabinet also should be offset from the optical axis 168a of the projection lens 168. Accordingly, the field angle increases in proportion to the amount of offset, which in turn increases not only the size of the projection lens 168, but also the angle of incidence of light on a Fresnel lens of the screen. Thus, flare or stray light is increased on the periphery of the screen, and the display images have poor quality.
Moreover, the field angles with respect to the most peripheral portions (four corners) of the screen differ from one another, resulting in nonuniform resolution or brightness on the screen.
Therefore, a projection system using right projection (non-offset) is suitable for the rear-projection display apparatus.
In contrast, a configuration that can achieve right projection by using a DMD has been proposed (e.g., JP 2001-166118 A).
According to this configuration, a TIR (total internal reflection) prism consisting of two or three pieces of prism is arranged between a projection lens and a DMD. Then, total reflection generated between the air gaps of each piece of prism is utilized to achieve right projection.
However, one side of a projected image can be blurred if there is nonuniformity in the air gaps of the TIR prism, so that extremely strict accuracy is required. Moreover, the TIR prism is a very expensive component and thus increases the cost of the whole apparatus.